L-Sorbose is one of the naturally occurring ketohexoses, which is contained in juice of the rowan tree, etc., and it is an important substance as a raw material in vitamin C synthesis. L-Sorbose is produced by fermentation wherein D-sorbitol is oxidized by microorganisms, for example, bacteria of the genus Gluconobacter.
According to a conventional fermentation production of L-sorbose, a yield of conversion of the raw material, D-sorbitol, into L-sorbose is at highest about 93%, and there are formed considerable amounts of by-products such as 5-ketofructose, D-fructose, 2-ketogluconic acid, etc. In order to reduce the raw material cost of vitamin C, it is requested to increase the yield of conversion of D-sorbitol into L-sorbose more than that in a conventional process and to inhibit formation of the above by-products to as little as possible.
In our European Patent Application No. 233050 (A2), there is disclosed that L-sorbose can be produced in a higher yield from D-sorbitol by microbiological oxidation using a microorganism which belongs to the genus Gluconobacter and which is decreased in the ability to grow with D-sorbitol as the single carbon source compared with that of its parent strains.
By the way, when the concentration of D-sorbitol in a culture medium goes up to over about 5% during and after the growth phase of the microorganisms, they are inhibited due to both high concentrations of the substrate (D-sorbitol) and the product (L-sorbose), which results in delay of the rate of oxidation. Accordingly, it is necessary to control the concentration of D-sorbitol below about 5%. However, when the concentration of the substrate is taken as a rate-determining step, there is a problem that the formation of by-products (fructose, 2-ketogluconic acid, etc.) is increased.
In addition, in this type of a production method, sometimes, a culture exhaust gas containing oxygen in a high concentration is recovered by a compressor, and it is circulated and reused in a culture liquid to save resources. In this case, carbon dioxide gas generated by respiration of the microorganisms is accumulated in the exhaust gas and, when the concentration of carbon dioxide gas goes up to over 10%, the growth rate of the microorganisms and the rate of oxidation are strongly inhibited. Accordingly, in a conventional method, there is employed carbon dioxide gas removing means using an adsorbent such as sodium hydroxide, activated charcoal, etc. However, when such means are employed, there are many problems such as increase in the production cost because of necessity of an adsorption column, an adsorbent and the like and further necessity of maintenance thereof.
Japanese Patent Kokoku No. 61-5709 discloses a method and an apparatus for culturing microorganisms such as fungi. In this publication, a gas enriched with oxygen is prepared from air by adsorbing nitrogen therein, and is used to control the partial pressure of carbon dioxide gas in a culture tank as well as to control the concentration of dissolved oxygen in a culture liquid.